Tailoring MXene Thickness and Functionalization for Enhanced Room-Temperature Trace NO2 Sensing

Highlights Gas-phase functionalization of X-MXene (X = –F, –OH, –O, –Br, –I) films crafted from sub-100 nm thin MXene flakes for highly sensitive NO2 sensors. I-MXene-based senor exhibited significant sensing performances toward trace NO2 at room temperature. The hydrophobicity, larger atomic size, lower electronegativity, and reduced shielding of -I contribute to the excellent sensing enhancement of I-MXene. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01316-x.

In this study, a total of 16 experiments were conducted to identify the most suitable condition for preparing few-layer MXene.The experimental conditions, including etching type, stirring time, autoclave temperature, and time, were listed in Tables S1-S16) for each method.SEM analysis for each sample obtained through all the methods was conducted, and the results are presented in Figs.S1 and S2 only for the methods that yielded more distinct and observable interlayer spacing.Fig. S1 specifically highlights the methods that yielded larger interlayer spacing and achieved MXene with a stacked-layer thickness of less than 1 µm.The most promising results were achieved when the MAX phase was stirred in HF (48%) for 36 h and then transferred to a simple autoclave, heated at 150°C for an additional 36 h (Fig. S1f, Fig. S2, and Table S15).This indicates that stirring alone did not significantly etch the A-layer from the MAX phases, resulting in approximately 500 nm thick layer-stacks in the MXene flakes (Fig. S1a and Table SI-1-2).However, transferring the stirred sample to a normal autoclave led to a reduction in thickness to around 150 nm, indicating the importance of applied pressure for effective etchant penetration (Fig. S1f, Fig. S2, and Table S15).Based on these results, a high-pressure stainless-steel autoclave was used, applying both stirring and pressurizing processes simultaneously.This strategic approach further increased etchant penetration between layers and enhanced interaction with a larger number of flakes, thereby achieving uniform layer-stacked thicknesses of about 50 nm throughout all MXene flakes even prior to the exfoliation process (Fig. S1(g-i)), indicating significant removal of the remaining Al.Subsequently, the product underwent 7 hours of tip-sonication and was washed using centrifugation.The sediment collected from the bottom and upper part of the centrifuge tube revealed a stack-layer thickness of about 1~2 µm (Fig. S1j) and <100~600 nm (Fig. 1a and Fig. S3(a-f), respectively.This confirmed that the hybrid HF-Hydrothermal synthesis condition played a critical role in effectively controlling MXene thickness, providing a promising approach for producing few-layer MXene materials with enhanced properties.

Nano-Micro Letters
Afterward, the sediment collected from the upper part of the centrifuge tube was dispersed in water and diluted to a 10-fold dilution compared to the as-collected sediment concentration (Fig. S4a).The diluted MXene sample was mixed with methanol and transferred dropwise into a beaker containing chloroform solvent, with a Si-wafer positioned at the bottom.Due to the immiscibility between methanol and chloroform created an interface where the few-layer MXene flakes (<100 nm) self-assembled, aligning their size with that of the substrate on the upper surface of the chloroform solvent (Fig. S4b).Subsequently, excess chloroform was removed, and the film level was adjusted to match the substrate.Finally, the substrate was lifted and subjected to a drying process at 100 °C for approximately 30 min to ensure the complete dryness of the MXene's film, revealing a thickness of <50 nm (Fig. 1b and Fig. S3a-c).The main steps involved in this process are illustrated in Schematic.

S3 Synthesis of Cl-MXene via molten salt approach
The Cl-MXene was prepared using a CuCl2 molten salt approach.This method did not involve high-pressure and stirring treatments, resulting in a smaller interlayer separation, as shown in Fig. S6a.The XRD pattern showed a slight shift in the (002) plane from 13.95° to 12.25 degrees (Fig. S6b), confirming reduced interlayer spacing.

S7 Role of functionalization on the electrical and environmental properties of MXene
In this comprehensive study, we investigated the resistivity and conductivity of different MXene samples, including as-prepared MXene, Br-MXene, and I-MXene, as given below.Our findings revealed distinct conductivity values for each variant.For I-MXene, the conductivity was measured to be 449.9640029S/m, while Br-MXene exhibited a conductivity of 394.29067S/m, and as-prepared MXene showed a conductivity of 341.320226S/m.Interestingly, recent studies have reported that de-functionalized MXene exhibits higher conductivity compared to functional group-based MXene.This observation can be attributed to the trapping of electrons by functional groups in the MXene channel (Ti3C2).Upon de-functionalization, these trapped electrons are released and reintegrated into the Ti3C2 structure, resulting in higher conductivity.Consequently, MXenes with high electronegative functional groups, such as -O, -OH, and -F, are expected to exhibit lower conductivity due to their higher tendency to trap electrons from the Ti3C2 MXene.
Consistent with these findings, our study demonstrated that MXenes with -O, -OH, and -F-based functional groups exhibited lower conductivity compared to MXenes with lower electronegative terminal groups such as -I and Br-.This correlation highlights the influence of the terminal group's electronegativity on electron trapping and conductivity in MXene materials.These results not only contribute to a better understanding of MXene properties but also pave the way for tailoring MXenes with specific functional groups to achieve desired electrical characteristics for diverse applications in electronics, energy storage, and sensors.

Conductivity calculation
For I-MXene: Moreover, the stability of both I-MXene and as-prepared MXene was evaluated in ambient and aqueous environments.In the ambient environment, I-MXene exhibited reasonable stability for up to 80 days, with a 176% change in resistance.However, beyond the 150 th day, a substantial increase in resistance exceeding 203,882% was observed, indicating significant instability.On the other hand, as-prepared MXene showed an extremely high change in resistance, reaching 99,613% by the 90th day, with no response observed beyond 110 days.In the aqueous environment, I-MXene exhibited an initial increase in resistance of about 172% during the first week, followed by more dramatic changes of 25,762% and 255,665% in the second and third weeks, respectively.Eventually, in the fourth week, no response was detected, suggesting complete oxidation.On the other hand, as-prepared MXene demonstrated significant increases in resistance, with a change in response reaching 16,903% in the first week and 186,385% in the second week.However, no response was observed in the third week, indicating weak oxidation.Notably, the high hydrophobicity of I-MXene contributed to its enhanced oxidation stability compared to as-prepared MXene in both ambient and aqueous environments, offering a promising solution for improving the long-term performance and reliability of MXene-based materials.

Fig. S1
Fig. S1 SEM images showing the layer-stacked thickness of the obtained MXene, with values less than 1 µm.(k) Layered-stack thickness obtained using various employed approaches

Fig
Fig. S6 (a) SEM iamge of Cl-MXene prepared via molten salt approach and (b) its XRD pattern.

Fig. S12 Fig. 15
Fig. S12 The resistance variation of both As-prepared and I-MXene over time (days) when stored in (a) ambient environment and (b) aqueous environment

Table S4
Method-4: Different time and 90 °C in the thermal ovenTable S5 Method-5: Different time and 150 °C in the thermal oven Table S6 Method-6: Different time and 180 °C in the thermal oven Table S7 Method-7: Different time and 60 °C in the thermal oven

Table S8
Method-8: Different time and 90 °C in the thermal ovenTable S9 Method-9: Different time and 150 °C in the thermal oven Table S10 Method-10: Different time and 180 °C in the thermal oven

Table S12
Method-12: Different time and 90 °C in the thermal ovenTable S13 Method-13: Different time and 150 °C in the thermal oven Table S14 Method-14: Different time and 180 °C in the thermal oven